The present invention relates to a process for the deposition by epitaxy of a doped material, as well as to an apparatus making it possible to perform this process.
It is more particularly used in the production of electronic or microelectronic components formed from several superimposed thin films, which are in particular semiconductive and which are differently doped. The main semiconductor materials used are silicon or III-V materials, such as gallium arsenide. The invention makes it possible to produce ultra-high frequency diodes, heterojunction lasers and very fast transistors, such as silicon--metal--silicon or SMS transistors, or permeable base transistors (base in the form of a gate).
The thin semiconductive films are generally produced by epitaxy on a monocrystalline semiconductor substrate. The presently known epitaxy methods are liquid phase epitaxy (LPE), chemical vapour phase epitaxy (CVD) and molecular beam epitaxy (MBE).
Among these different epitaxy methods, molecular jet epitaxy has intrinsically higher performance levels, particularly for obtaining very thin films of a few nanometers, with a perfectly planar surface and a very abrupt interface. A molecular beam epitaxy process and apparatus are described in an article by Y. OTA, published in J. Appl. Phys, 51(2), February 1980, pp 1102-1110 entitled "Silicon molecular beam epitaxy with simultaneous ion implant doping".
In this method, in a tight enclosure are formed molecular fluxes or beams of the material to undergo epitaxy, e.g. silicon and the heated substrate is subject to the action of these molecular beams, as well as to the action of a doping particle beam.
The substrate heating temperature is relatively low (600.degree. to 850.degree. C.) compared with other deposition processes and particularly chemical vapor phase deposition process (1000.degree. to 1200.degree. C.). At this low temperature, there is no diffusion of the doping particles and consequently it is possible to produce differently doped superimposed layers with an extremely abrupt doping profile, which is indispensable for fast components.
However, this deposition procedure causes a certain number of problems, particularly with respect to the doping of semiconductor layers. Thus, for certain doping particles, e.g. arsenic, phosphorus antimony, etc, used for a p-type doping of the silicon, a very disturbing phenomenon appears. In particular, the doping material is evaporated in the form of molecular of type X.sub.4 (As.sub.4, P.sub.4, etc) and at the relatively low substrate heating temperature of 600.degree. to 850.degree. C., there is a doping gas vapour tension, which as the effect that a majority of the doping molecules which touch the surface of the semiconductor layer undergoing epitaxy, do not integrate with said layer and instead return to the doping gas.
It has been possible to determine a "bonding coefficient" which is in practice approximately 10.sup.-4, i.e. one doping atom is integrated with the layer undergoing epitaxial growth for 10,000 atoms on the surface thereof. This low "bonding coefficient" makes it necessary to inject into the epitaxy enclosure, in which there is a very high vacuum, a large amount of a doping gas, which is incompatible with the need of maintaining an ultra-high vacuum.
One of the presently proposed solutions for increasing the "bonding coefficient" on the substrate to undergo epitaxy consists of polarizing said substrate. This substrate polarization makes it possible to attract ionized doping particles which then strike the substrate surface. Unfortunately, this bombardment of ionized particles leads to damage to the epitactic layer by producing numerous crystalline faults in said layer, which can bring about the formation of a substantially amorphous layer (sputtering effect).